As large-scale integration (LSI) progresses toward higher integration density and higher processing speed, reduction of pattern rules also progresses rapidly. In the background of this rapid progress of downscaling, projection lenses having a higher numerical aperture (NA) are being developed, resist composition performance is being improved, and light sources are being shifted to shorter wavelengths.
General use of a resist composition for a krypton-fluoride (KrF) examiner laser (248 nm) started in a 0.3-μm process, and this has been applied to mass-production of a 0.13-μm rule. The shift to a shorter wavelength from a KrF excimer laser to an argon-fluoride (ArF) examiner laser (193 nm) makes it possible to reduce the design rule to 0.13 μm or less, but because resins such as a novolak resin or a polyvinyl phenol resin, which have been used in the past, have strong absorption of light that is near 193 nm near 193 nm, they cannot be used as a resist base resin. To secure transparency and sufficient dry-etching resistance, an acryl resin and a cycloolefin-based alicyclic resin were evaluated, and as a result, mass-production of a device using ArF lithography was realized.
In the 45-nm node device, the wavelength of the exposure-light source was shortened, and thus F2 lithography of 157 nm became a candidate for next-generation lithography. However, F2 lithography has many problems, such as: increased cost of scanners due to the introduction of expensive CaF2 single crystals in a projection lens; the need to change the optical system due to the extremely poor durability of soft pellicles, thereby necessitating hard pellicles; and decreased etching resistance of the resist. Thus, postponement of F2 lithography and early introduction of ArF immersion lithography were proposed, and 45-nm node devices using ArF immersion lithography are being produced on a mass scale. For mass-production of 32-nm node devices, a double patterning process using a sidewall spacer is used, but the process is long and complicated, which is a problem.
What is wanted for a device after 32 nm is not an expensive, double-patterning process, but EUV (extreme ultraviolet) lithography of 13.5 nm, the resolution thereof being improved by shifting the exposure light to a wavelength that is shorter by more than one digit than previous lithography technology, and thus development of EUV lithography of 13.5 nm is progressing. In EUV lithography, the power of a laser is weak, and the amount of light decreases according to the attenuation of reflective mirror light, and thus the intensity of light reaching a wafer surface is low. Thus, to acquire throughput with low light intensity, development of a highly sensitive resist is urgently needed. However, if the sensitivity of the resist is enhanced, there is a problem of deterioration of both resolution and edge roughness (LER: line-edge roughness, and LWR: line-width roughness), and thus a trade-off relationship between increased sensitivity and deterioration in resolution and in edge roughness has been pointed out.
The EUV resist has a problem in that because of its high sensitivity it is easily affected by contaminants in the atmosphere around it. An amine quencher usually is added to a chemically amplified resist to ease the effect of amine contamination in air, but the amount of the amine quencher added to the EUV resist is a small fraction of the amount of the amine quencher added to an ArF resist. Accordingly, the EUV resist tends to form a T-top shape by receiving the effect of the amine from the resist's surface. The formation of a top coat on a resist's upperlayer is effective in preventing the effect of contaminants in the surrounding atmosphere. In an early type of a chemically amplified resist for a KrF excimer laser based on a t-BOC (tertiary-butoxycarbonyl)-protected polyhydroxy styrene, to which an amine quencher was not added, the use of a top coat was effective in preventing the effect of contaminants in the surrounding atmosphere. Even in an early stage of ArF immersion lithography, a top coat was used to prevent elution of an acid generator into water, thereby preventing formation of a T-top configuration that would result from such elution. Here, also in the EUV lithography process, to form a top coat on the upper layer of a resist was proposed as described in Non-patent Document 1. By forming a top coat, environmental resistance is improved, and outgas from the resist film is reduced.
An EUV laser of DPP (discharge-produced plasma) and LPP (laser-produced plasma) emits, in addition to 13.5-nm-wavelength light that is used for patterning, a weak broadband light having a wavelength of 140 nm to 300 nm (out-of-band, OOB). The intensity of this broadband light (hereinafter “OOB light”) is weak, but the amount of the light's energy cannot be neglected because of its wide wavelength range. A Zr filter is provided to an EUV microstepper to cut the OOB light, but this decreases the light intensity. In an EUV scanner in which, to improve throughput, the light intensity is not allowed to decrease, it is possible to not use this filter. Non-patent Document 1 shows the superiority of forming a top coat to shield the resist's upper layer from the OOB light.
Many kinds of materials have been proposed for use as a resist top coat in ArF immersion lithography. Among the materials proposed, Patent Document 1, discussed below, discloses a top coat composition containing a polymer of a repeating unit of styrene having a 1,1,1,3,3,3-hexafluoro-2-propanol group, which is supposed to be unpractical due to very high absorption of light that is at a wavelength of 193 nm.
Patent Document 2 points out that, in the case of a top coat for immersion lithography: (1) a solvent for a top coat dissolves the surface of the resist film, thereby causing mixing between the top coat and the resist film, and this in turn causes film loss of a resist pattern after the film has been developed; (2) especially, when an alcohol solvent is used, a great amount of film loss occurs; and (3) an ether solvent is effective in inhibiting film loss. An example of a polymer that is soluble in ether solvents is a polymer that has a 1,1,1,3,3,3-hexafluoro-2-propanol (HFA) group, as described in Patent Document 2. However, the fluorine atoms in a polymer strongly absorb EUV light, and thus if a HFA-containing polymer is used as the top coat of a resist's upper layer, the sensitivity of the resist is lowered after patterning, which is a problem.
Non-patent Document 1 reports that (1) in the case of a positive-type resist film, when an entire wafer is exposed, the width of the lines in the peripheral region of a shot becomes narrower due to OOB light leaked from neighboring shots; and (2) if a top coat that absorbs light having a wavelength of 200 nm to 300 nm is applied on a resist film, the variation of the pattern size within a shot can be reduced. Patent Document 3 proposes a top coat of a hydroxyl-styrene or a cresol-novolak resin applied as a solution of alcoholic solvents such as 4-methyl-2-pentanol, 2-methyl-2-pentanol, or isopropyl alcohol, 3,3,5-trimethyl-1-hexanol.
Patent Document 4 provides a method that (1) uses a resist for ArF immersion lithography, based on a polymethacrylate substituted by an acid labile group to form a positive pattern by developing with alkaline, and (2) forms a negative pattern by developing by using an organic solvent. That document also shows that using a protective film for developing with an organic solvent reduces the amount of substances eluted in the immersion liquid and improves water-droplet sliding properties.
Patent Document 4 also discloses a method of forming a negative pattern by developing with an organic solvent under EUV exposure. In ArF exposure, arranging a dot-pattern mask to form a hole pattern with a negative resist has an advantage in that it is possible to use high-contrast light. However EUV light, which has a wavelength shorter than that of ArF light by one digit or more, is not advantageous in the contrast in arranging the dot-pattern mask. Nonetheless, EUV exposure is advantageous only in that the resist sensitivity thereof can be enhanced because the number of photons under EUV exposure is larger than under ArF exposure.